In conventional, non-hydrogenative catalytic cracking it is well known that efficiencies, as measured by gasoline/coke ratios, depend to a large extent upon the nature of the feedstock employed. Feedstocks rich in nitrogen compounds, sulfur compounds and/or heavy polycyclic condensed-ring aromatic hydrocarbons, tend to give relatively high coke and light gas yields and relatively low gasoline yields. Another significant factor is the boiling range distribution of the feed. Other things being equal, low boiling hydrocarbons generally require more severe conditions to maintain a given conversion to gasoline than do higher boiling hydrocarbons. If a feedstock boils over a wide range of say 400.degree. to 1000.degree. F., it is difficult to select a cracking temperature which is optimum for all hydrocarbon fractions in the feed. If high cracking temperatures are utilized in order to maintain adequate conversion of the lower boiling fractions, the higher boiling fractions then tend to produce inordinate amounts of coke and light gases. Conversely, if low temperatures are employed in order to optimize conversion of the heavy fractions and minimize coke formation, then conversion of the lower boiling fractions is reduced, resulting in low overall conversions per pass and high recycle rates.
In view of the above difficulties, considerable effort has been devoted in the past to upgrading and optimizing catalytic cracking feedstocks. Catalytic hydrofining has been suggested as a means of reducing the nitrogen and sulfur contents of such feeds, and also for partially hydrogenating heavy polycyclic hydrocarbons. In general it has been found that catalytic hydrofining is very successful for reducing sulfur and nitrogen contents, but when its use is extended to the partial hydrogenation of polyaromatics, overall desirable results appear to be limited to cases where the initial feedstock contains no more than a minor proportion of material boiling above about 800.degree. F. For high boiling feedstocks, it is found that the severe hydrofining conditions required for denitrogenation, desulfurization and polyaromatics hydrogenation, inherently bring about a substantial conversion of the material boiling above about 800.degree. F. to lower boiling hydrocarbons. This may in some cases be desirable, but in many cases it results in an undesirable production of large amounts of low octane gasoline in the hydrofiner, and also in an uneconomical consumption of hydrogen for the hydrocracking of heavy material which could more economically be converted in the catalytic cracker--and to a higher octane gasoline product.
Moreover, the conversion of 800.degree. F.+ material during hydrofining results in a catalytic cracking feedstock much enriched in mid-boiling-range hydrocarbons (400.degree.-800.degree. F.), but still containing a "tail" of 800.degree. F.+ material. For practical purposes this tail fraction must be removed from the cracking charge stock so that conditions can be optimized therein for conversion of the lower boiling materials, which conditions would otherwise result in excessive conversion of the tail fraction to coke. In summary therefore, it is difficult to utilize catalytic hydrofining alone to effect adequate upgrading and optimizing of cracking feedstocks containing substantial amounts of material boiling above about 800.degree. F.
It has also been suggested in the art that unconverted oils boiling above the gasoline range resulting from catalytic hydrocracking operations utilizing conventional metal-promoted silica-alumina cogel type catalysts, can also be utilized as catalytic cracking charge stocks. Although these unconverted oils do in many instances form advantageous cracking feedstocks, it has been found that in general they suffer from the same limitations as do the severely hydrofined oils discussed above. The amorphous cogel type hydrocracking catalysts tend, like hydrofining catalysts, to convert selectively the heavy portions of the feed, giving a product rich in mid-boiling-range hydrocarbons, but lean in heavy ends. Here again, economical operation of the catalytic cracker generally requires removal of the heavy ends, thus again effectively limiting upgraded charge stocks to an end-point of about 800.degree. F.
A primary objective of the present invention is to provide a process for upgrading catalytic cracking charge stocks which contain substantial proportions, e.g., at least about 20 weight-percent, of material boiling above 800.degree. F., in such manner as to minimize hydrogen consumption, the production of low octane hydrogenated gasolines, and to provide a denitrogenated, desulfurized and partially hydrogenated cracker charge stock which includes a sufficient amount of heavy material boiling above about 800.degree. F. to justify the use of cracking temperatures aimed more at the conversion of the heavy fraction under relatively non-coking conditions, while still maintaining a relatively high overall conversion per pass to gasoline. Conventional catalytic hydrocracking cycle oils (from which the 800.degree. F.+ fraction has not been removed) when cracked at the same low severity levels give low conversions and high recycle rates; and if cracking temperatures are raised to achieve adequate conversions, coking rates and dry gas yields are materially increased.
According to our invention, the initial raw heavy feedstock is first subjected to catalytic hydrofining to a limited extent necessary to reduce the sulfur and nitrogen contents to the desired level, but insufficient to complete the desired hydrogenation of polyaromatics, and final upgrading of the product is carried out under mild hydrocracking conditions over a specific type of hydrocracking catalyst which is effective for the partial hydrogenation of polyaromatics, but due to its pore size limitations, selectively hydrocracks material boiling in the 600.degree. - 800.degree. F. range, but effects minimal hydrocracking of the 800.degree. F+ material. The resulting product fraction boiling above about 600.degree. F., and even in some cases the entire product fraction boiling above an initial temperature as low as 400.degree. F., can then be catalytically cracked at relatively low temperatures selected for optimum conversion of the 700.degree. F.+ , or 800.degree. F.+ material while maintaining desirably high overall conversion rates and low coke yields. This result can be achieved primarily because the unique hydrocracking step has materially reduced the amount of 600.degree. - 800.degree. F. boiling range material in the feed while effecting a relatively insignificant reduction in the content of 800.degree. F.+ material.
Moreover, a fortuitous aspect of the invention is that the minor proportion of C.sub.6 - 400.degree. F. gasoline synthesized in the hydrocracking step has an unusually high octane number, a result which is believed attributable to the aromaticity of the initial feed from which it was derived. The hydrocracked product fraction boiling in the 400.degree. - 600.degree. F. range is also highly aromatic in nature and may hence be advantageously recycled to the hydrocracking step, or separately hydrocracked to produce additional high octane hydrocracked gasoline. Alternatively, since the aromatics in this fraction are predominantly monoaromatics, it can advantageously be included in the feed to the catalytic cracker without materially decreasing conversion levels at a given coke yield.